The present invention relates to an equalizer, an equalization method, and a transmitter. Particularly, the present invention relates to an equalizer and equalization method for compensating attenuation of high frequency components caused by the skin effects of-transmission line conductors, the dielectric loss of a printed board material and the like in a high speed interconnection system in between a plurality of printed boards constituting a large capacity communication device such as an IP router or a cross connector or a large scale information processing device such as a super computer, and to a transmitter.
As an example, FIG. 1 shows a constitution of a large capacity communication device that transmission and reception of an electric signal are performed between different printed boards (hereinafter referred to as “PKG”) 1a and 1b via a back board (hereinafter referred to as “BWB”) 2. The electric signal to be transmitted is transmitted from a transmitting circuit 11a to a receiving circuit 12b, and a transmission line 20 comprises pattern wirings 20 formed on PKGs 1 and BWB2 and BWB connectors 21 for electrically connecting the specific pattern wirings 20 formed on the PKGs 1 and BWB2 to each other. The total length of the pattern wirings 20 extending from the transmitting circuit 11a to the receiving circuit 12b is several tens of centimeters to several meters.
In a range where the bit rate of an electric signal transmitted through the pattern wirings 20 exceeds 1 Gbps, frequency dependency occurs in the transmission properties of the electric signal, due to an increase in transmission loss caused by the skin effects of conductors forming the pattern wirings 20 and an increase in dielectric loss of insulators constituting the printed boards 1 and BWB 2 in addition to the electric resistance of the conductors. More specifically, a received code error occurs due to intersymbol interference caused by so-called waveform distortion which is significant attenuation of the high frequency components of the transmitted signal. FIG. 2 shows an example of a received waveform with significantly attenuated high frequency components.
To deal with such a problem, a technique referred to as preemphasis which comprises performing equalization at the time of transmission has heretofore been known. The preemphasis technique is described in A. Fiedler et al., “A 1.0625 Gbps Transceiver with 2× Oversampling and Transmit Signal Pre-emphasis,”, Proc. IEEE Int. 1 Solids-State Circuits Conf., Digest of Technical Papers, IEEE Press, Piscataway, N.J., 1997, pp. 238-239, or M. Fukaishi, K. Nakamura, M. Yotsuyanagi, et. al., “A 20-Gb/s CMOS Multi-Channel Transmitter and Receiver Chip Set for Ultra-High Resolution Digital Display”, 2000 ISSCC Digest of technical Papers, San Francisco, pp. 260-261, February 2000. FIG. 3 shows a measurement example of the waveform of a signal equalized at the time of transmission.
In this preemphasis method, an equalizing circuit for emphasizing high frequency components is provided to the transmitting circuit 11a so as to emphasize the high frequency components of the spectrum of a signal to be transmitted at the time of transmission of the signal and compensate for attenuation of the high frequency components in the transmission line.
Thereby, the influence of intersymbol interference on a received signal is reduced, and transmission and reception free from code errors are performed.
In the above method of performing equalization at the time of transmission, an equalization parameter must be set for each transmitting circuit from the following two reasons.
(1) Attenuations of high frequency components in the pattern wirings 20 take different values. From the viewpoint of the constitution of the device, the lengths of the pattern wirings 20 are not the same. The pattern wirings 20 are ones on the PKGs which are connected to the adjacent BWB, and one on the BWB which is connected to the PKGs at both ends. For example, the length of the pattern wiring may range from 10 cm to 100 cm. Thus, the length of the pattern wiring varies within a certain range, and according to its transmission distance, the attenuation of the high frequency components varies within a certain range.
(2) An optimum equalization parameter solely exists for a certain length of the pattern wiring. When emphasis of the high frequency components by equalization is excessive, an eye opening becomes small while a maximum amplitude becomes large, thereby reducing a receiving margin disadvantageously.
Referring to FIG. 4, brief description will be made of the constitution of a related equalization parameter setting circuit. There is no means for automatically knowing the length of the pattern wirings connecting the transmitting circuit 11a and the receiving circuit 12b to each other. Therefore, different equalization parameters S1301 must be set for the transmitting circuits 11a after assembly of the device.
In the above-mentioned related art, setting of the equalization parameter S1301 every time the power is turned on has been avoided generally by use of storage means such as a nonvolatile memory. However, it must be manually set at least once after assembly of the device. This is because pattern wirings 20 to which a certain transmitting circuit 11a is connected are determined for the first time when PKGs 1a and 1b are connected to BWB 2 at the time of the assembly of the device and, before that, there is no means for knowing the length of the pattern wirings 20, so that the equalization parameter S1301 cannot be determined.
As a result of making such a manual setting, labor costs required for the setting and reservation of the stock for a time period required for the adjustment arise, thereby causing an increase in the price of the device.